. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 304 CLEAVAGE (SEGMENTATION) AND BLASTULATION figs. 144G, H; 145J-L.) This condition is found in the monkey, human, pig, rabbit, etc. (2) On the other hand, in certain marsupials, such as the American opos- sum, Didelphys virginiana, and the BraziUan opossum, Didelphys aurita, the inner cell mass is much less prominent during earlier stages of the blastocyst. In these species it is indicated merely by a thickened aggregation of cells at one pole of
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 304 CLEAVAGE (SEGMENTATION) AND BLASTULATION figs. 144G, H; 145J-L.) This condition is found in the monkey, human, pig, rabbit, etc. (2) On the other hand, in certain marsupials, such as the American opos- sum, Didelphys virginiana, and the BraziUan opossum, Didelphys aurita, the inner cell mass is much less prominent during earlier stages of the blastocyst. In these species it is indicated merely by a thickened aggregation of cells at one pole of the blastocyst (fig. 147E-G). (3) In the marsupial or native cat of Australia, Dasyurus viverrinus, cleav- age results in an early blastocyst in the form of a hollow sphere of rounded cells. As the blastocyst expands, the cells increase in number and become flattened to form a thin layer of cells apposed against the shell membrane without an apparent inner cell mass or embryonic knob (fig. 148A-C). A conspicuous feature of cleavage and early blastocyst formation in the marsupials should be emphasized. For in this group, the early blastomeres apparently use the framework of the zona pellucida as a support upon which they arrange themselves. As a result, the blastocoelic space of the blastocyst. Fig. 148. Early blastular conditions of the marsupial cat of Australia, Dasyurus viverrinus. (After Hill, '10.) (A) Early blastula. (B) External view of blastocyst, mm. in diameter. The cells are becoming flattened and finally reach the condition shown in (C). (C) Section of wall of blastocyst. mm. in Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Nelsen, Olin E. (Olin Everett), b. 1898. New York, Blakiston
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